1. Field of the Invention
The present invention relates to a method and apparatus for fabricating liquid crystal displays, and particularly to a method and apparatus for fabricating liquid crystal displays preventing a gravity mura problem from occurring.
2. Description of the Prior Art
A liquid crystal display (LCD) has advantages of lightness, low power consumption, less radiation and applied to various portable electronic products such as notebook computers and personal digital assistants (PDAs). In addition, LCD monitors and LCD televisions are gaining popularity as a substitute for traditional cathode ray tube (CRT) monitors and televisions.
The LCD panels are usually viewed at a standing-up position, and gravity will change the pressure balance of the liquid crystal inside the LCD panel. “Standing-up position” means a position such that the direction along panel thickness is in the horizontal direction or approximately perpendicular to the earth-gravity direction. Please refer to FIG. 1, which is a schematic diagram of a standing-up conventional LCD panel. The LCD panel 100 has two glass substrates 102 and 104, and liquid crystal 106 is filled between the glass substrates 102 and 104 and is encapsulated by a sealing area 108. When the LCD panel 100 is in a standing-up position, three kinds of forces, which are shown as F1, F2, and F3 in FIG. 1, act onto the contact face of the liquid crystal 106 in the bottom region of the LCD panel 100 and the glass substrate 102 or 104. The arrows indicate the directions of F1, F2, and F3. The first force F1 is due to a pressure difference between the liquid crystal 106 in the LCD panel 100 and the ambient. The second force F2 is due to the capillarity of the liquid crystal 106. The third force F3 is due to the gravity force of the liquid crystal 106.
When the sum of the pulling forces F1 and F2 is larger than or equal to the pushing force of F3, the liquid crystal 106 in the LCD panel 100 is in a pressure balanced condition, and the images can be normally displayed. If the sum of the pulling forces F1 and F2 is smaller than the pushing force of F3, a gravity mura, or gap mura, will happen at the bottom region of the standing-up LCD panel 100, and seriously worsen the image quality. Accordingly, to prevent from the mura defects, a relation of F1+F2>F3 must be maintained, in which F2 and F3 each is approximately a fixed value, and F1 remarkably varies with the variation of temperature.
Please refer to FIG. 2. FIG. 2 is a schematic cross sectional diagram showing a conventional LCD panel 200 horizontally positioned. The LCD panel 200 comprises two glass substrates 202 and 204 assembled by a sealing area 208. A liquid crystal cavity (LC cavity) 210 is defined by a plurality of spacers 212 between the glass substrates 202 and 204 inside the sealing area 208. Liquid crystal 206 is filled in to the LC cavity 210. The spacers 212 are used to maintain a predetermined gap between the two glass substrates 202 and 204. At room temperature, the surface of the two glass substrates 202 and 204 is flat and the pressures inside and outside the LC cavity are substantially same, that is, force F1 shown in FIG. 1 is equal or approximate to 0. The weight of the liquid crystal 206 in the LC cavity 210 is defined as a standard liquid crystal filling weight.
However, at a high temperature, as shown in FIG. 3, because liquid crystal 206 has a thermal expansion coefficient more than those of the glass substrate 202, 204 and spacers 212, the glass substrate 202, 204 will bulge outward. The internal pressure of the LC cavity 210 is higher than the external pressure, that is, the force F1 shown in FIG. 1 is less than 0. In such situation, LCD panel 200 cannot comply with the relation of F1+F2>F3, that is, the LCD panel 200 will have a gravity defect in a high temperature environment. Such defect is especially significant in the manufacturing of large size LCD panels.
Most conventional end seal processes for display panels adopt a multistage pressure increase/decrease method, however, the panels manufactured tend to have a gravity defect and other mura problems as mentioned above, especially for a large size display panel.
For example, U.S. Pat. Nos. 6,086,443 and 6,208,405 disclose a sealing method for panels and attempt to eliminate the mura on LCDs or form uniform liquid crystal cell gap. Please refer to FIG. 4 showing the time-pressure curve during the end-seal process of U.S. Pat. No. 6,086,443. During initial T1 time, the pressure gradually increases from P1 to P2. During the next T2 time, the residual amount of the liquid crystal filled in the liquid crystal cell is discharged by maintaining the pressure P2 constantly. Next, with maintaining the pressure P2, a sealant is applied to the injection hole. During the next T3 time, the pressure decreases from P2 to P3, and during the next T4 time, the sealant is flattened with maintaining the pressure P3 constantly. Finally, after the end-seal sealant is hardened by the ultraviolet (UV) irradiation, the pressure is eliminated. However, in the multi-stage pressure end-seal process, the panel is not heated, which is only heated when compressing the two glass substrates and structurally attaching the sealant and the substrates together in order to define a cell cavity, and the gravity defect and other mura problems are not solved.